1. Field of the Invention
This invention relates to synchronizing and timing systems, and, more particularly, to synchronizing and timing systems for high speed testing of electronic circuits by electro-optical test systems.
2. Prior Art
Increased speeds provided by improvements in integrated circuits and devices using emitter-coupled logic ECL, gallium arsenide, and complementary-MOS technologies has created a need for production-testing and circuit-characterization test equipment which can test integrated circuits and devices operating in excess of 200 MHz. A test system operating above 1 gigaHz, discussed in the copending patent application incorporated herein by reference, generates electronic stimulus test vectors having a NRZ data rate of 1.2 Gb/s and uses an electro-optic effect, the Pockels effect, to measure the input and output signals of a device under test DUT.
Various techniques are available in the prior art for synchronizing and timing electro-optic test systems.
One technique uses an optical sampling pulse both as the timing reference for the generation of electronic test signals and as the optical sampling pulse for measurement of the resultant electronic signals. U.S. Pat. No. 4,446,425 to Valdmanis et al. and U.S. Pat. No. 4,681,449 to Bloom et al. each describe systems which use light pulses from a laser source to optically generate electronic pulses from photodiodes. The pulses are then coupled directly to a Pockels cell. Phasing, or delay, of an electronic pulse with respect to a light pulse is accomplished by mechanical variation of the optical path travelled by a light pulse prior to its impinging upon a photodiode. Such mechanical variations makes scanning measurements very slow and subject to misalignment of precision optical paths.
Another technique locks a laser to a frequency synthesizer. The frequency synthesizer is used as the time base for generation of electronic stimulus signals The synthesizer is tuned to a harmonic frequency of the optical pulse repetition frequency plus a small offset frequency. Consequently, the laser pulse applied to the electro-optical detector appears to slowly scan the repeating electronic signal as a sequential sampling pulse. This technique has several drawbacks. The stimulus signal frequency is constrained to be at or very near a harmonic frequency of the laser pulse repetition frequency, which severely limits the frequency resolution range of such a system. Such a system can only operate in a sampling mode and an optical pulse cannot be rapidly positioned on a repeating stimulus signal.